Resistors in the form of relatively thin films of resistive material deposited between electrical contacts are well known in the art. These resistors typically include a metal component and may further comprise an oxide or a semiconductor component. The resistance of such thin film resistors are often adjusted by removing, etching, abrading, etc., portions of the resistor material.
When a thin film resistor is positioned on a cylindrical substrate, the prior art indicates that it can be trimmed by forming a helical groove, via laser erosion, in a conductive film which coats the resistor (U.S. Pat. No. 4,566,936 to Bowlin), or by cutting a plurality of helically oriented, narrow grooves in the resistance film to decrease the amount of resistive material between opposing contacts (U.S. Pat. No. 3,509,511 to Soroka).
When a film resistor is configured on a planar substrate, a number of techniques are taught in the prior art for trimming the resistance value. In one configuration, a film of resistive material is located between the ends of a pair of conductive terminals so as to create a "top hat", with the terminals defining the hat brim. Removing the film material progressively from either the bottom or the top of the top hat modifies the amount of resistive material (and thus the resistance value) between the contacts. Such trimming methods are shown in U.S. Pat. Nos. 3,573,703 to Burks et al., and 4,163,315 to Neese.
A number of prior patents teach the trimming of a planar resistor by initially positioning an erosion instrument (e.g. an electro-erosion head, laser beam, etc.) outside the limits of the resistor and slowly bringing it within the boundaries of the resistor so that a kerf is created in the resistive material. Such a technique, and variations thereof, can be found in U.S. Pat. Nos. 3,889,223 to Sella et al., 3,947,801 to Bube, 4,159,461 to Kost et al., 4,352,005 to Evans et al., 4,443,782 to d'Orsay, 4,551,607 to Moy, 4,647,899 to Moy, and 4,785,157 to Gofuku et al. In the latter patent, in addition to cutting a kerf into the resistive material, the resistance is irradiated in chosen areas to change local resistance characteristics. The common feature to the above mentioned patents is that each teaches that the erosion of the resistance material commences from outside and then proceeds inwardly into the resistive material.
The prior art also teaches that the resistance value of a film resistor can be modified by maintaining the erosion instrument totally within the limits of the resistive material. For instance, in U.S. Pat. Nos. 4,205,297 and 4,301,439, both to Johnson et al., a film type resistance is provided with a central contact and an exterior contact located at the resistor's periphery. The resistor is trimmed by cutting a spiral pattern in the resistive material to, essentially, elongate the resistive path between the central contact and the exterior contact. In U.S. Pat. No. 4,582,976 to Merrick, a trimming technique is disclosed wherein a substantially rectangular-shaped film resistor is trimmed by removing an internal portion of the resistive material to create an opening therein that is parallel to the long dimension of the resistor.
Similar types of trimming techniques have also been applied to planar capacitive structures (e.g. see U.S. Pat. Nos. 3,394,386 to Weller et al., 3,402,448 to Heath and 3,597,579 to Lumley).
Of the above indicated trimming techniques, the most widely used is laser-based. Those systems commence the trim action by positioning a laser beam outside of the boundary of the resistor, and then traversing the beam across the edge and into the body of the resistor. While this method is efficient in terms of obtaining a maximum change in resistance, it presents a number of problems when applied to recent semiconductor/thin film resistor structures. Such structures typically comprise a silicon masterslice with a plurality of layers of personalizing metallization and intervening quartz, nitride, or other insulating ceramic materials disposed therebetween. Two, three or more of such complex layers can often be found on a masterslice, with thin film resistors disposed within such structures. Often, resistors are placed on the uppermost surface and are passivated with an additional layer of a sputtered quartz material.
It is preferred that the thin film resistors be trimmed after the semiconductor device is completely fabricated (i.e. after the final layer of quartz passivation has been deposited over the resistors). Because the passivating quartz layer seals the surface of the thin film resistors, care must be taken to assure that the amount of material vaporized by an incident laser beam is such that the thus created vapor pressure does not rupture the quartz layer. It is also desirable that trimming be accomplished using a method that produces a wide range of very precise and reliable resistance values for the least amount of trim action and within the least amount of resistor "real estate." Further, the trimming action should be accomplished rapidly, with the highest efficiency and with the least expenditure of laser energy.
These objectives are partially accomplished by making the thickness of the thin film resistor the minimum that will provide the required resistance value. The trim action is preferably adjusted so as to provide the desired change of resistance in the shortest period of trim time. Traditionally, a laser cutting procedure which proceeds from the outside boundary of a thin film to the inside has been utilized. In thin film resistors passivated with an overcoat of quartz, the use of such a trim procedure can cause defects at the entrance of the trim cut into the resistor material. The primary problem appears to be that the trim process does not always remove all of the material at the edge. The material which remains can, in some cases, act as an electrical bridge element across the cut and present a reliability exposure.
A possible solution to the edge defect problem would be to increase the power of the laser above that which would otherwise be required. In addition to potentially injuring the overlying quartz layer, such action might tend to anneal, and thus change the characteristics of, underlying semiconductor structures. Another possible solution might be to provide added trims at the entrance to the cut, however, this would require additional time for the trim action and reduce production throughput.
Accordingly, it is an object of this invention to provide a thin film resistor structure and trim technique that yields resistors with a wide range of accurate and reliable resistor values and is achievable on resistor structures of minimum "real estate".
Another object of this invention is to provide a method for trimming thin film resistors that does not induce undesirable structural effects in the vicinity of the irradiated region.
It is a further object of this invention to provide a method for trimming a planar thin film resistor covered by a rigid overlayer.